1. Field of the Invention
This invention relates to lens moving devices for use in focusing or zooming photographic lenses.
2. Description of the Prior Art
There have been known mechanism for transmitting rotative movement of the actuating ring to axial movement of the focusing member or zoom members by using cam sleeves.
Also, many attempts have been made to employ new materials for making up those parts of the lens mounting which are associated with such mechanism. For example, in the past, the main constituent members of the lens mounting, such as the body tube, lens cell, cam sleeve, intermediate or helicoid sleeve, distance adjusting ring and zoom actuator ring were made of metalic materials such as aluminum alloys and brass. Since these metals are very expensive and the machining cost is also high, the sum of the material and machining costs account for a considerably large proportion of the price of the product.
To eliminate the above-described problem of the metallic material, increasing attempts of employing plastic materials in some of the constituent parts of the lens mounting begin to be made. The use of the metallic material and the plastic material in the constituent members of the lens mounting gives rise to bad influences on the optical performance and the mechanical strength, because, as the ambient temperature changes, the difference between the coefficients of linear expansion of both materials takes the form of a discrepancy of the actual value of axial movement of the lens member from the ideal value.
To overcome this problem, there has been a previous proposal for contructing the mechanism in such a way that the side edges of the camming slot are formed to tapered shape, and the cam follower is urged by a spring member to abut on the aforesaid tapered surface in Japanese Laid-Open Utility Model Application No. Sho 57-170873.
This prior known art has, however, the following problem. As the guiding groove of the body tube is crossed over the camming slot of the cam sleeve at a point in which the guide pin on the lens cell is fitted, for the amount (angle) of rotation of the cam sleeve is reduced to effect an equivalent total axial movement of the lens, the guiding groove must be not linear in parallel with the optical axis, but arcuate. For this purpose, if the body tube and the cam sleeve are intended to be made of resin material by molding techniques, the metal mold is required to have undercut portions for the groove and slot of the tube and sleeve. Therefore,
(1) The arcuate slot and groove of the molded body tube and cam sleeve are formed unavoidably with gradient walls necessary to allow for the core of the metal mold to be pulled out from the formed piece.
(2) As the thickness of the body tube and the cam sleeve for the ordinary diameter of the lens mounting is about 2 mm, there is a critical condition that the allowable range of radial lengths of the undercut portion be less than 2 mm. This implies that all the thickness cannot be undercut when molding.
For these reasons, with the mechanism having the body tube in which the axially movable or rotatable sleeve is fitted, the movable lens cell fitted in the sleeve, and the guide pin radially extending from the cell through the camming slot of the sleeve, when the side edge of the camming slot is made tapered by undercut molding techniques, a problem is produced that a sufficient strength cannot be obtained due to a limitation on the thickness of the axially movable or rotatable sleeve resulting from the critical condition of the undercut molding.